What is, is and what isn't, is not.
For to be is to exist,
and not to be is to exist not.
Which amounts, I assure you,
to a very sturdy knot.
Stan Sýkora, part of Perpetuum Mobile, Arese, May 1995
Part of the Perpetuum Mobile physics rhymes:
Illustrates the basic ontological impass of the
absence of a primary definition of nonexistence and
the fact that its secondary definition as a negation
of existence relies on faulty circular logic.